To increase the recording density of optical disks, generally the light source wavelength has to be reduced or the NA of the objective lens has to be increased. For DVD optical disks, a wavelength of 650 nm and NA of 0.6 have been used, but it would be possible to increase the recording density by using a blue emitting light source with a wavelength of 400 nm and increasing the NA of the objective lens up to 0.85. With such an optical head, however, a slight error in optical disk substrate thickness would result in a large spherical aberration.
For example, in the case of the first-described DVD optical system, a substrate thickness error of 10 μm results in an RMS spherical aberration of about 0.01λ, but with the latter optical system conditions, the same substrate thickness error of 10 μm would result in a spherical aberration of about 0.1λ which is 10 times larger.
Spherical aberration correcting optical systems have been proposed for correcting such spherical aberration. For example, Japanese Patent Application No. 2000-131603 proposes a method in which an afocal optical system is constructed using two lenses, a convex lens and a concave lens, and the spacing between the lenses is varied to correct spherical aberration.
In this optical system, however, if the convex lens or the concave lens is moved along the optical axis to correct spherical aberration, there arises a first problem, that is, the light utilization efficiency varies. This will be described in detail below.
A semiconductor laser or the like is used as the light source for an optical head. The far field intensity pattern of a semiconductor laser resembles a Gaussian distribution. That is, the intensity is the highest on the optical axis, and decreases with the distance from the optical axis. Generally, if it is attempted to increase the amount of light incident on an objective lens, the intensity of light at the periphery or rim intensity of the effective diameter of the object lens decreases. If the intensity of light at the periphery or rim intensity of the effective diameter decreases markedly, the diameter of the spot focused by the object lens increases. Conversely, if it is attempted to obtain a uniform intensity distribution within the effective diameter of the object lens, the efficiency of gathering light from the semiconductor laser drops. In this way, how much of the light emitted from the semiconductor laser is to be gathered into the objective lens is a crucial parameter that affects the performance of the optical head.
However, when one of the lenses in the afocal optical system is moved to correct spherical aberration, the angle of the light incident on the object lens changes when viewed from the semiconductor laser as the light source, and hence there has been the first problem in that the light gathering efficiency and the diameter of the spot focused by the object lens change.
Further, in the case of a disk comprising two layers of different thicknesses for increased storage capacity, the second layer is thicker than the first layer. Recording/reproduction on the second layer is performed using the light passed through the first layer. Recorded portions and non-recorded portions are unevenly distributed in the first layer, and this affects the recording/reproduction characteristics of the second layer. Hence, there arises a second problem, that is, it is desirable that the effective NA for the second layer be made larger.
In the case of a thick disk, coma aberration that occurs when the disk tilts increases. Further, the light absorption is larger than in the case of a thin disk, hence a third problem.